98 SEA-SHORE LIFE 
sort of wood excepting teak. It burrows through the softer parts of 
the wood, completely honey-combing the logs so that the knots and 
hard parts project. Submerged timber decreases in diameter at the 
rate of about an inch per year clue to the ravages of this pest. Fortu- 
nately, however, it does not occur in any considerable numbers much 
below tide level. Submerged wood must be sheathed in copper or 
frequently painted with creosote or poisonous substances, in order 
to prevent its destruction. 
CRABS 
Brachyura. 
The crabs are the highest of the Crustacea. The head and 
middle part of the body are covered with a shell which is usually 
broader than long. The abdomen is small, and is folded forward, 
fitting into a groove on the lower surface of the animal. In the 
male the abdomen is narrow, and has only two pairs of append- 
ages, while in the female it is much broader, and has four pairs of 
appendages which serve to carry the eggs (see Fig. 47, Page 79). 
The two pairs of feelers are small, and the eyes are mounted on 
movable stalks which may usually be folded back into appropriate 
sockets. The mouth appendages are broad and flat so as to 
cover the opening of the mouth itself. ‘The first pair of legs 
have pincers, while the following four pairs lack pincers but are 
formed for walking or clinging; the hindermost being often paddle- 
shaped and used in swimming. ‘The plumed gills are contained in 
side chambers enclosed by the shell of the middle part of the body. 
Being thus protected from drying, many of the crabs may live for 
long periods of time on land, their gills remaining moist. Crabs 
live in both salt and fresh water, and at all depths. Some are active 
swimmers or crawlers while others are sedentary or even parasitic. 
They are scavengers, devouring almost any dead and decaying 
animal or plant. Despite their unclean habits, many species are 
highly esteemed as food, and crab fisheries are of great commer- 
cial importance. When hatched from the egg, crabs are totally un- 
hike their adult form, and are said to be in the Zowa stage. The 
head and middle part of the body are covered with a delicate shell 
which gives rise to four long, sharp spines. One of these is directed 
